Tundra Polygons: The Key to Arctic Diversity

Mostly high-center tundra polygons on the north slope in the Arctic National Wildlife Refuge, Alaska. This is as an area where most of the polygons, which usually feature a small pond in the middle, have drained.

As I wrote in the last post, tundra polygons are one of the primary features of the arctic landscape. I mean, let’s face it, the tundra of Alaska’s arctic coastal plain (and much of the rest of the arctic for that matter), is lacking in geographic relief. Thus, the seemingly inconsequential low ridges, dips, troughs, and up-raised areas of polygons are disproportionally important to tundra habitats.

They are so important that these features create some of the highest biodiversity areas of the arctic tundra. This is no surprise if you’ve ever walked across polygon tundra. One step you’ll be standing atop well-drained Dryas flowers on a narrow soil ridge or high-center polygon, when another few feet you’ll step ankle-deep into wet-sedge tundra of a low-center polygon. Step forward a bit more and your mis-placed foot may drop hip-deep in the open water of an ice-wedge trough. It’s impossible habitat to walk with any kind of rhythm, as you constantly adjust your stride to splash through water, step over a 12 inch ridge, or stride easily across the dry patches.

That varied terrain is reflected in the biodiversity of the polygons. As noted above, the vegetation can change dramatically meaning that the species richness of plants rises as well. Invertebrates, both terrestrial and aquatic also find ample habitat. Phillip Martin, a biologist at the USFWS in Fairbanks, while doing his masters work on the Canning River Delta of the Arctic National Wildlife Refuge, found that the wet troughs between polygons produced a high proportion of the insect biomass found in the tundra. Unsurprisingly, he also found higher diversity and abundance of nesting and migrant shorebirds in the same habitat.

Over the Canadian border on the Mackenzie River Delta, researcher C.L Gratto-Trevor found a similar pattern. In the mid-90s she found high numbers of nesting shorebirds of a dozen species preferred the low-center polygons where wet-sedge tundra dominated. There, she found as many as 82 pairs per square km.

If similar numbers of birds turn up in such habitat across the arctic, (and I expect they do), we are talking about millions and millions of nesting shorebirds. Low-center polygons, may be the most important feature of the arctic landscape when it comes to nesting and post-breeding shorebirds.

Low-center polygons may be one of the most important features of the arctic landscape when it comes to nesting and post-breeding shorebirds… And that habitat, ain’t faring so well.

And that habitat, ain’t faring so well.

A few years ago I was working in the oil fields surrounding Prudhoe Bay surveying nesting birds. Along the dusty roads of the oil fields and nearby Dalton Highway, prevailing winds carry the blown dust out over the nearby tundra. In these areas, warmed disproportionately by the presence of the sunlight-absorbing dust, the polygons have drained. Deep, empty troughs create a miniature canyonland of polygons. The polygons themselves have lost their diversity, with the wet sedges replaced by upland plants.

The dust effect above is restricted to the areas downwind from the few roads that cross the arctic, but global climate change has no such limitation. In a mid-90s paper in the journal Ecology, Stuart Chapin and collaborators found that experimental warming of plots near the Toolik Lake Field Station in arctic Alaska resulted in lost diversity, drying, and increased shrub growth. And it wasn’t just the experimental plots. The surrounding tundra, was already showing similar signs of change resulting from broad-scale warming in the arctic.

It’s scary but unavoidable. As the arctic warms, we are going to see the low-center polygons steadily replaced by high-center dry polygons. Shorebirds are going to be restricted to remaining wet areas, and I suspect population declines will follow.